To suppress the emission of carbon dioxide gas from automobiles, high-strength steel sheet is being used to reduce the weight of automobile body. Further, to secure the safety of passengers, not only soft steel sheet, but also high-strength steel sheet is being made much use of for automobile body. In addition, to reduce the weight of automobile body in the future, new demand is rapidly rising for raising the level of usage strength of high-strength steel sheet.
However, when bending deformation is applied to high-strength steel sheet, because of the high strength, the “spring back” phenomenon of the shape after the work tending to deviate from the shape of the forming jig and return in the direction of the shape before the work and the “wall camber” phenomenon of the planes of the side walls ending up as surfaces having curvature due to elastic recovery as a result of bending-rebending during work occur.
Therefore, in a conventional automobile bodies, the steel used has mainly been limited to high-strength steel sheet of less than 440 MPa strength. For automobile body, it is necessary to use high-strength steel sheet of more than 490 MPa strength to reduce the weight of the body. Despite this, there is no high-strength steel sheet with little spring back and wall camber and a good shape fixability.
Without having to say it, rising the shape fixability after working high-strength steel sheet or soft steel sheet of less than 440 MPa strength is extremely important in raising the shape precision of automobiles, household electric appliances, and other products.
Some of the inventors disclosed in WO 00/06791 a ferritic thin steel sheet with a ratio of the {100} plane and {111} plane of at least 1 for the purpose of improving the shape fixability, but the patent document has no description of reduction of the wall camber. Therefore, the X-ray intensity ratio in the orientation component group of {100}<011> to {223}<110> to the X-ray random diffraction intensity ratio and those in the orientation components of {100}<011> are not described either in the patent document.
Further, some of the inventors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-64750, as technology for reducing the amount of spring back, a cold-rolled steel sheet wherein the reflected X-ray intensity ratio of a {100} plane parallel to the sheet plane is controlled to 3 or more. However, this cold rolled steel sheet is characterized by specifying the x-ray intensity ratio at the outermost surface in the sheet thickness, so is steel sheet completely different from the present invention.
Further, some of the inventors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-363695 and Japanese Patent Application No. 2002-286838 (Japanese Unexamined Patent Publication (Kokai) No. 2004-124123) a low yield ratio high-strength steel sheet excellent in shape fixability and a method of producing the same.
Compared with these inventions, the present invention studies the production conditions whereby a more excellent shape fixability is realized and production conditions whereby both a shape fixability and workability are obtained.
That is, the inventors discovered that for this, control of the texture and control of the anisotropy of ductility are extremely important and, as result of intensive study, discovered optimal control conditions satisfying these requirements.